The invention concerns a steering system, particularly for a vehicle that has a wheel steering and an articulated steering, with a steering unit, a first steering motor arrangement, a second steering motor arrangement and a selection valve, which connects the steering unit with either the first steering motor arrangement or the second steering motor arrangement. Further, the invention concerns a method for steering a vehicle, in which a steering unit activates a first or a second steering, particularly a wheel steering or an articulated steering.
A conventional steering system and a method are shown in DE 35 43 054 A1. By means of the selection valve, the driver can direct the hydraulic fluid controlled by the steering unit to the first steering motor arrangement, which acts upon steered wheels, for example the front wheels of a tractor, a building site vehicle or another self-propelled working machine. After changing over the selection valve at the end of the activation of the wheel steering, the steering unit can also be connected with the second steering motor arrangement, which acts upon a converting kit. In this connection, the vehicle has two parts, which are articulately connected with each other via the converting kit. When the two vehicle parts are brought to an angle position in relation to each other by means of the converting kit, relatively small radii can be driven with the vehicle.
A similar system is shown in U.S. Pat. No. 4,565,257. In this system, the steered wheels cannot only be turned in the same direction as the articulation of the vehicle, but also in the opposite direction, so that with articulated front part the vehicle can still drive straight forward.
Finally, U.S. Pat. No. 4,802,545 shows a steering system for a vehicle with wheel steering and articulated steering.
With such vehicles, the driver selects the suited mode of steering according to what is needed. However, the need may change during the driving or even during a steering movement. The changing over from one mode of steering to another may partly cause substantial deviations in the steering behaviour. These deviations are not only unpleasant for the driver. They may also cause dangerous situations.
Therefore, the principal objects of this invention are to provide increased driving comfort and safety. These and other objects will be apparent to those skilled in the art.
The steering system of this invention has a steering unit with a variable displacement and a control device which adjusts the displacement in dependence of the connection established by a selection valve.
With vehicles of the kind known, the consumption of the first steering motor arrangement, which acts upon steered wheels, is different from that of the second steering motor arrangement, which acts upon the converting kit. Usually, the steering motor arrangement for the converting kit is larger and therefore needs more hydraulic fluid to perform the same directional change of the vehicle. The quantity of hydraulic fluid, which is supplied in connection with a movement of the steering handwheel (or another steering device), is determined by the displacement of the steering unit, usually through a measuring motor section. When a steering unit is used, in which a first rotary slide is displaced by the steering handwheel in relation to a second rotary slide, to permit the flow of hydraulic fluid, which is supplied by a pump, through the steering unit, the measuring motor unit makes the second rotary slide follow in dependence of the passing oil quantity, thus a turning of the wheels or an articulation is proportional to the angular position of the steering handwheel. When now the displacement of the steering unit is chosen in dependence of whether the wheels are steered or the vehicle is articulated, the same movements of the steering handwheel cause the same steering behaviour of the vehicle with regard to curve radii.
The steering unit has two different displacements. As only two different steering methods, namely preferably articulated steering and wheel steering, are available, it is sufficient to let the steering unit work with two different displacements. Each displacement can then be adapted to the desired application.
The steering unit has a measuring motor section with two measuring motors, a change-over valve being provided, which either deactivates a measuring motor or lets it work additionally to the other measuring motor. A steering unit of this kind, bearing the name of OSPD, is available from Dan-foss A/S, Nordborg, Denmark. It is also described in DE 195 11 501 A1. When changing over the change-over valve, the displacement of the steering unit is increased or decreased in a simple way.
A delay device is provided, which sets a delay period between an activation of the change-over valve and an activation of the selection valve. An activation of the change-over valve simultaneously with the activation of the selection valve may cause relatively large jerks in the vehicle, which are uncomfortable for the driver. When, however, a time delay is introduced between the two change-over procedures, such jerks do not occur, and the comfort is of the driver is not adversely affected. The delay period is not longer than 500 milliseconds, and usually is 200 milliseconds. Such short delay periods have no disadvantageous influences on the driving behaviour of the vehicle. However, they are sufficient to avoid the jerk or the shock that may otherwise occur in connection with the change-over. It is also advantageous to have the delay period be a function of a rotary speed of a steering handwheel. Thus, the higher the rotary speed, the longer can be the delay period be chosen. As described above, a smaller displacement can be chosen for the wheel steering than for the articulated steering. Thus, the displacement can be adapted to the two different steering forms in such a way that the same turning of the steering handwheel will always result in the driving of the same curve radii. At least when only one of the two steering forms is activated, the driver has the same steering feeling. Deviations may occur, when with an articulated vehicle also the wheels are turned. However, such a case demands a deliberate action by the driver, in connection with which he must anticipate a different steering behaviour of the vehicle. In the xe2x80x9cnormal casexe2x80x9d, when the driver selects a steering form from straight forward driving, the result will be conditions equal or at least similar to those described above.
When changing over from one steering form to the other, a delay period takes place between the change-over of the displacement and the change-over of a connection between the steering unit on the one side and the wheel steering or the articulated steering on the other side. When the change-over between the two steering forms and the change of the displacement occur at the same time, a jerk may sometimes appear, which is felt as an unpleasant shock by the driver. When, a delay period takes place, this jerk is avoided.
When changing over from the articulated steering to the wheel steering, the displacement of the steering unit is changed first, and then the connection is changed over. The jerk or shock mainly appears when changing over from the articulated steering to the wheel steering. The delay period increases the comfort of the driver.
The delay period is selected in dependence of a steering speed. The faster the vehicle is steered, that is, the higher the speed with which the directional change takes place, the longer is the delay period. On the other hand, this means that in connection with slow directional changes the delay period can be very short. The steering speed is derived from a rotary speed of a steering handwheel.